This invention relates to ultrasonic diagnostic imaging systems and, in particular, to ultrasonic diagnostic imaging systems which produce synthetic transmit focus scanlines at a high rate of acquisition.
Conventional ultrasonic diagnostic imaging systems utilize dynamic focus during beam formation to dynamically vary the focal range of received ultrasonic echoes as the echoes are received. This capability is made possible by the ability to continually change the delays applied to echoes received from different elements of a transducer array as the echoes are received, thereby continually changing the focal range of the transducer array""s receive aperture. The effective curvature of the receive aperture is constantly made more gradual to progressively focus the transducer array at increasing ranges as echoes are received from ever-increasing depths following a transmit wave.
Such a dynamic focus capability is not possible during transmit, however. A transmitted beam can be focused at only one range by selectively delaying the application of transmit pulses across the aperture, with elements at the center of the array transducer experiencing delayed excitation in relation to elements located toward the lateral extremes of the transmit aperture. Once the transmit wave has been launched, it is not possible for the ultrasound system to modify or adjust its focus. Various techniques have been tried to effect a focal region at different or more extended depths such as focusing different elements of the array at different depths, combining multiple transmit pulses in a single transmission, encoding different frequency components to focus at different depths, and lateral deconvolution. All of these techniques have met with mixed success, complexity and/or compromise.
A conventional technique for dealing with the single focal characteristic of the transmit beam is what is known as multi-zone focus. In multi-zone focus each beam location is interrogated multiple times, with each transmission focused at a different transmit depth. During reception echoes are received around the focal range of each unique transmit beam, thereby acquiring segments of a complete receive scanline from different depth ranges. The segments are then spliced together to form a complete scanline for an image. A problem which is inherent in multi-zone focusing is the time required to scan an image field and produce an image, since each scanline must be interrogated multiple times in order to form a single scanline. It would be desirable to be able to improve the transmit focal characteristics of an ultrasound system but without the need to greatly reduce the frame rate of display.
In accordance with the principles of the present invention, received ultrasound signals with differing transmit steering directions and focal characteristics are processed to synthesize the characteristics of an extended transmit focal zone. In a preferred embodiment the processing is done on coherent echo signals received by multiline reception by varying the signal delay, weighting, or both, then combining the signals to form scanlines with extended focal characteristics. A preferred embodiment combines both lateral and focal interpolation to produce image lines at a high frame rate of display. The technique of the present invention can also be applied to a single focus scanline to improve spatial accuracy.